Are you absolutely certain that you refer to available local storage - or available iCloud storage? These are independent.
iPad Storage:
Settings > General > iPad Storage
iCloud Storage:
Settings > [Your Name / AppleID] > Manage Account Storage
These support pages may prove to be helpful:
About storage on your device and in iCloud – Apple Support
What's the difference between device storage and iCloud storage? – Apple Support
Manage your iCloud storage – Apple Support
How to check the storage on your iPhone, iPad and iPod touch – Apple Support
Clear Other storage on your iPhone or iPad - Apple Support
With a better understanding of your local device and iCloud storage, if outlined here, contributors may be better able to provide specific guidance.